This invention relates to a sorbent tube trace sample releasing apparatus. This invention is particularly useful for the injection of a sample into gas chromatographs and other analytical instruments.
It is well known in the art that gas chromatography requires a constant flow of a stream of inert gas (called the carrier gas) be maintained through the partition column, the gas stream serving primarily to transport the chemical sample to be analyzed through the column and hence to the detector at the exit end. For the column to retain optimum efficiency and for repeatibility of results, it is important that the column be kept purged with a purging gas stream during introduction of the sample.
In conventional gas chromatographs samples are injected through a rubber septum into the gas stream with a needle syringe the samples usually being in the liquid form. However, for trace samples collected, or "preconcentrated" in adsorbent traps such as, for example, when ambient air is monitored for trace contaminants (see for example Canadian Patent No. 1,201,646, issued Mar. 11, 1986, inventors A. H. Lawrence and L. Elias), the conventional method of sample injection is not suitable because a solvent extraction step is required which is not only time consuming but results in the loss of too much sample when the sample is a trace sample.
A number of approaches have been attempted as alternatives to the conventional septum injection method, such as, for example:
Canadian Patent No. 903,519, dated June 27, 1972, T. Johns, discloses a sample injection apparatus for introducing a predetermined fluid sample into a fluid stream in a predetermined way so that such problems as baseline drift caused by slow vaporization of the sample can be avoided.
U.S. Pat. No. 3,482,450, dated Dec. 9, 1969, R. J. Harris, Sr, and R. J. Harris, Jr., discloses a sample inlet system which includes a septum and is concerned with the smooth introduction and carriage of samples into the carrier gas stream of apparatus such as gas chromatographs.
U.S. Pat. No. 3,566,698, dated Mar. 2, 1971, W. M. Sheppard, discloses a septumless inlet for chromatographs directed to overcoming the problem of the limited life of septums. Sheppard's inlet has a casing containing a spring loaded shuttle having a bore, closed at the downstream end, for receiving the open end of a syringe. The shuttle has a neck portion containing a cross bore for the escape of sample existing from the syringe in a carrier gas. When the shuttle is in a retracted position, an O-ring seal around the neck portion seals a sample outlet from the cross bore. When the shuttle is urged in a forward direction the O-ring seal seals the shuttle to the casing rearwardly of the cross-bore so that sample can be carried by the carrier gas along a sealed path to the chromatographs. While the inlet of Sheppard is useful, there are no provisions for preventing the escape of sample rearwardly around the shuttle while the shuttle is being moved forwardly from the retracted position. Consequently Sheppard's inlet is not suitable for use when the sample is a trace sample and only a very small loss of the sample as it is being transferred from the syringe to the gas chromatograph is allowable.
U.S. Pat. No. 4,294,117, dated Oct. 13, 1981, H. B. User et al., discloses a sample charger for a gas chromatograph wherein a sample in a sample holding capsule is entrained in a carrier gas and is transferred to an upstream end of an adsorption tube in an annular passage. The sample is free to flow along the annular passage in a cavity around the outside of the adsorption tube and so the charger of B user et al is not suitable for use with trace samples.
U.S. Pat. No. 4,612,019, dated Sept. 16, 1986, M. L. Langhorst, teaches that collected constituents on an adsorbent or absorbent material may be desorbed, for example, with a suitable solvent and subsequently analyzed by well-known analytical methods such as liquid or gas chromatography. Langhorst does not teach an apparatus that is suitable for releasing a trace sample from a sorbent tube for analyses in a gas chromatograph.
U.S. Pat. Nos. 3,735,640, dated May 29, 1973, Chizhov et al, and 4,414,857, dated Nov. 15, 1983, show that it is well known to use valves to control carrier gas flow to a gas chromatograph, but they do not teach an apparatus that is suitable for transferring a trace sample from a sorbent tube for analyses in a gas chromatrograph.
There is a need for a valved apparatus suitable for transferring a trace sample from a sorbent tube to a chromatograph wherein the loss of sample during the transfer is negligible.
According to the present invention there is provided a sorbent tube trace sample releasing apparatus for sample analysis, comprising;
(a) a tubular casing having an outlet at an end downstream with respect to the flow of a trace sample from the tubular casing, for connection to a sample analyzing apparatus, the tubular casing having a sorbent tube receiving portion extending downstream from an upstream, sorbent tube inlet end of the casing, a valve seating in the tubular casing and at the downstream end of the sorbent tube receiving portion, and a purging gas inlet to the casing interior and adjacent to a downstream side of the valve seating,
(b) a valve seal spring loaded in an upstream direction against the valve seating, for displacement by a blunt end of the sorbent tube,
(c) a closure valve connected to the purging gas inlet,
(d) sealing means, adjacent an upstream side of the spring-loaded valve seal, for slidably sealing a downstream end of the sorbent tube in the sorbent tube receiving portion with the valve seal against the valve sealing,
(e) heating means for heating the casing, the valve seal and the sorbent tube thereby thermally releasing a sample from a downstream end of the sorbent tube in the casing, whereby, in operation,
(f) with the sorbent tube displacing the valve seal from the valve seating, the heating means heating the casing, the valve seal and the sorbent tube, thereby thermally releasing a trace sample from the sorbent tube, and carrier gas conveying released trace sample to a sample analyzing apparatus connected to the downstream end of the casing, the sealing means being adjacent an upstream side of the spring-loaded valve seal permits substantially only the heated valve seal, valve seating and casing interior downstream thereof to be contacted with released sample, so that loss of the trace sample is minimal and is encountered substantially only in a relatively small portion of the casing interior adjacent the outlet, so that
(g) for purging the apparatus it is only necessary for purging gas to passed into the purging gas inlet with the valve seal closed against the valve seating, so that only the relatively small portion of casing interior is purged.
In some embodiments of the present invention the valve seating is a chamfered portion of the interior of the casing 1 and enlarged in the downstream direction, and the valve seal has a truncated-cone-shaped sealing surface which seals against the chamfer.